This invention relates to a system for controlling the rotation of a screw which feeds a molten material under pressure in an injection molding machine. More particularly, the invention relates to a rotation control system for the screw of an injection molding machine adapted to enable smooth and accurate control by using a numerical control unit (NC unit).
In the working of plastics, particularly most thermoplastic elastomers such as vinyl chloride resin, it is strongly desired that productivity be improved by reducing the energy needed for such working and that a uniform product quality be obtained. Accordingly, in recent years such devices as injection molding machines having a control unit incorporating a computer for accurate machining control have been put into practical use. FIG. 4 is a view showing the control system arrangement of an injection molding machine employed in the prior art. This conventional injection molding machine control system will now be described with reference to FIG. 4. In FIG. 4, numeral 1 denotes a hopper for holding chips of a thermoplastic elastomer such as vinyl chloride, and numeral 2 designates a screw for feeding the chips contained in the hopper 1 in the direction of a cylinder 2'. Due to rotation of the screw 2, the chips are fed to the cylinder 2' and are melted by the application of heat. The molten material, which is supplied to and fills the cylinder 2' and a nozzle 3, is injected into a mold 4 by movement of the screw 2 in the direction of the nozzle 3. Numeral 5 denotes a hydraulic motor for rotatively driving the screw 2, 6 a flowrate control valve for regulating the amount of oil supplied to the hydraulic motor 5, 7 a hydraulic cylinder for controlling back-and-forth movement of an injection shaft 2" (a shaft for moving the screw back and forth) inside the cylinder 2', and 8 a servo valve for controlling hydraulic pressure fed to the hydraulic cylinder 7. Numeral 9 denotes a tachogenerator for sensing the rotational speed of the screw 2 and for applying a screw rotation signal to a process control unit 15, described below. Numeral 11 represents an absolute encoder for sensing the axial position of the screw 2 and for producing a screw position signal applied to the process control unit 15. Numeral 12 designates a pressure sensor for sensing hydraulic chamber pressure of the hydraulic cylinder 7 and for producing injection pressure and back pressure signals applied to the process control unit 15. Numeral 13 denotes a cavity pressure sensor for sensing the pressure in a cavity of the mold 4 and for producing a detection signal applied to the process control unit 15.
The process control unit 15 has an internal computer, executes processing on the basis of input signals from each of the above-mentioned sensors and from temperature sensors provided on the mold 4 and heating cylinder, and outputs a servo valve control signal, flowrate control signal and temperature control signal.
In the conventional injection molding machine control system constructed as set forth above, the servo valve 8 is operated on the basis of the flowrate control signal and servo valve control signal produced by the process control unit 15 after the mold 4 is clamped and set, and the screw 2 is moved in the direction of the nozzle 3 so that the molten material filling the interior of the cylinder 2' is injected into the mold 4 from the nozzle 3. When the molten material is thus injected, the process control unit 15 executes control for dwell over a predetermined period of time, subsequently executes cooling control, retracts the screw 2 by operating the hydraulic cylinder 7 and effects control for unclamping the mold. After the workpiece molded by the mold is extracted, the mold 4 is reclamped, the hydraulic motor 5 is driven to rotate the screw, and the chips in the hopper 1 are fed into the cylinder 2' and melted. Thereafter, the injection operation is performed as described above to mold workpieces one after another.
In the conventional injection molding machine control system, the screw 2 is operated by the hydraulic motor 5 and hydraulic cylinder 7, so that control is implemented hydraulically. However, a hydraulic control system has a slow control response and a complicated mechanism, and is troublesome in terms of maintenance.
A system has recently been proposed in which rotation and movement of the injection molding machine screw are performed by electric motors and primary control is implemented by a computer.
However, various problems are encountered in control when the screw and injection shaft of the injection molding machine are driven by electric motors.
For example, since the chips in the hopper 1 are fed into the cylinder 2', a reaction force for the pressured feed of the chips acts upon the screw 2 when the screw is rotated. Since the screw 2 is rotated by the electric motor and a torque limit is imposed upon the motor, the screw 2 is moved backward by the reaction force when this force exceeds the torque limit. As a result, the chips are not supplied to the cylinder 2' in an accurate amount. Suitable means for solving this problem does not exist in the prior art.